feat(dds): реализован слой dds для bookings

- Зачем:
  - подготовлен учебный Star Schema слой для перехода от ODS к аналитике и витринам.
- Что:
  - добавлены 21 SQL-файл для DDS (DDL/LOAD/DQ) с SCD1/SCD2 и фактом `fact_flight_sales`.
  - добавлены DAG `bookings_dds_ddl` и `bookings_to_gp_dds`, а также smoke-тесты структуры DAG.
  - обновлены `sql/ddl_gp.sql` и документация (`README`, `docs/*`, `db_schema`) под поток `stg -> ods -> dds`.
- Проверка:
  - make test.

sql/dds/dim_airports_load.sql

@@ -0,0 +1,60 @@
+-- Загрузка DDS dim_airports: SCD1 UPSERT (UPDATE изменившихся + INSERT новых).
+
+-- Statement 1: UPDATE существующих записей (если атрибуты изменились).
+UPDATE dds.dim_airports AS d
+SET airport_name = s.airport_name,
+    city         = s.city,
+    country      = s.country,
+    timezone     = s.timezone,
+    coordinates  = s.coordinates,
+    updated_at   = now(),
+    _load_id     = '{{ run_id }}',
+    _load_ts     = now()
+FROM ods.airports AS s
+WHERE d.airport_bk = s.airport_code
+    AND (
+        d.airport_name IS DISTINCT FROM s.airport_name
+        OR d.city IS DISTINCT FROM s.city
+        OR d.country IS DISTINCT FROM s.country
+        OR d.timezone IS DISTINCT FROM s.timezone
+        OR d.coordinates IS DISTINCT FROM s.coordinates
+    );
+
+-- Statement 2: INSERT новых записей (MAX(sk) + ROW_NUMBER()).
+WITH max_sk AS (
+    SELECT COALESCE(MAX(airport_sk), 0) AS v
+    FROM dds.dim_airports
+)
+INSERT INTO dds.dim_airports (
+    airport_sk,
+    airport_bk,
+    airport_name,
+    city,
+    country,
+    timezone,
+    coordinates,
+    created_at,
+    updated_at,
+    _load_id,
+    _load_ts
+)
+SELECT
+    (SELECT v FROM max_sk) + ROW_NUMBER() OVER (ORDER BY s.airport_code)::INTEGER,
+    s.airport_code,
+    s.airport_name,
+    s.city,
+    s.country,
+    s.timezone,
+    s.coordinates,
+    now(),
+    now(),
+    '{{ run_id }}',
+    now()
+FROM ods.airports AS s
+WHERE NOT EXISTS (
+    SELECT 1
+    FROM dds.dim_airports AS d
+    WHERE d.airport_bk = s.airport_code
+);
+
+ANALYZE dds.dim_airports;